Compatibility of probes, catheters, cardiac pacemakers, implantable defibrillators and other types of active implantable medical devices with magnetic resonance imaging (MRI) and other types of hospital diagnostic equipment has become a major issue. If one goes to the websites of the major cardiac pacemaker manufacturers in the United States, which include St. Jude Medical, Medtronic and Boston Scientific (formerly Guidant), one will see that the use of MRI is generally contra-indicated with pacemakers and implantable defibrillators. See also:    (1) Safety Aspects of Cardiac Pacemakers in Magnetic Resonance Imaging”, a dissertation submitted to the Swiss Federal Institute of Technology Zurich presented by Roger Christoph Luchinger, Zurich 2002;    (2) “1. Dielectric Properties of Biological Tissues: Literature Survey”, by C. Gabriel, S. Gabriel and E. Cortout;    (3) “II. Dielectric Properties of Biological Tissues: Measurements and the Frequency Range 0 Hz to 20 GHz”, by S. Gabriel, R. W. Lau and C. Gabriel;    (4) “III. Dielectric Properties of Biological Tissues: Parametric Models for the Dielectric Spectrum of Tissues”, by S. Gabriel, R. W. Lau and C. Gabriel; and    (5) “Advanced Engineering Electromagnetics, C. A. Balanis, Wiley, 1989;    (6) Systems and Methods for Magnetic-Resonance-Guided Interventional Procedures, U.S. Patent Application Publication No. US 2003/0050557, Susil and Halperin et. al, published Mar. 13, 2003;    (7) Multifunctional Interventional Devices for MRI: A Combined Electrophysiology/MRI Catheter, by, Robert C. Susil, Henry R. Halperin, Christopher J. Yeung, Albert C. Lardo and Ergin Atalar, MRI in Medicine, 2002; and    (8) Multifunctional Interventional Devices for Use in MRI, US 2003/0050557, and its underlying provisional application Ser. No. 60/283,725.
The contents of the foregoing are all incorporated herein by reference.
However, an extensive review of the literature indicates that MRI is indeed often used with pacemaker, neurostimulator and other active implantable medical device (AIMD) patients. The safety and feasibility of MRI in patients with cardiac pacemakers is an issue of gaining significance. The effects of MRI on patients' pacemaker systems have only been analyzed retrospectively in some case reports. There are a number of papers that indicate that MRI on new generation pacemakers can be conducted up to 0.5 Tesla (T). MRI is one of medicine's most valuable diagnostic tools. MRI is, of course, extensively used for imaging, but is also used for interventional medicine (surgery). In addition, MRI is used in real time to guide ablation catheters, neurostimulator tips, deep brain probes and the like. An absolute contra-indication for pacemaker or neurostimulator patients means that these patients are excluded from MRI. This is particularly true of scans of the thorax and abdominal areas. Because of MRI's incredible value as a diagnostic tool for imaging organs and other body tissues, many physicians simply take the risk and go ahead and perform MRI on a pacemaker patient. The literature indicates a number of precautions that physicians should take in this case, including limiting the power of the MRI RF Pulsed field (Specific Absorption Rate—SAR level), programming the pacemaker to fixed or asynchronous pacing mode, and then careful reprogramming and evaluation of the pacemaker and patient after the procedure is complete. There have been reports of latent problems with cardiac pacemakers or other AIMDs after an MRI procedure sometimes occurring many days later. Moreover, there are a number of recent papers that indicate that the SAR level is not entirely predictive of the heating that would be found in implanted leads or devices. For example, for magnetic resonance imaging devices operating at the same magnetic field strength and also at the same SAR level, considerable variations have been found relative to heating of implanted leads. It is speculated that SAR level alone is not a good predictor of whether or not an implanted device or its associated leadwire system will overheat.
There are three types of electromagnetic fields used in an MRI unit. The first type is the main static magnetic field designated B0 which is used to align protons in body tissue. The field strength varies from 0.5 to 3.0 Tesla in most of the currently available MRI units in clinical use. Some of the newer MRI system fields can go as high as 4 to 5 Tesla. At the recent International Society for Magnetic Resonance in Medicine (ISMRM), which was held on 5 and 6 Nov. 2005, it was reported that certain research systems are going up as high as 11.7 Tesla and will be ready sometime in 2010. This is over 100,000 times the magnetic field strength of the earth. A static magnetic field can induce powerful mechanical forces and torque on any magnetic materials implanted within the patient. This would include certain components within the cardiac pacemaker itself and/or leadwire systems. It is not likely (other than sudden system shut down) that the static MRI magnetic field can induce currents into the pacemaker leadwire system and hence into the pacemaker itself. It is a basic principle of physics that a magnetic field must either be time-varying as it cuts across the conductor, or the conductor itself must move within a specifically varying magnetic field for currents to be induced.
The second type of field produced by magnetic resonance imaging is the pulsed RF field which is generated by the body coil or head coil. This is used to change the energy state of the protons and elicit MRI signals from tissue. The RF field is homogeneous in the central region and has two main components: (1) the electric field is circularly polarized in the actual plane; and (2) the H field, sometimes generally referred to as the net magnetic field in matter, is related to the electric field by Maxwell's equations and is relatively uniform. In general, the RF field is switched on and off during measurements and usually has a frequency of 21 MHz to 64 MHz to 128 MHz depending upon the static magnetic field strength. The frequency of the RF pulse for hydrogen scans varies by the Lamor equation with the field strength of the main static field where: RF PULSED FREQUENCY in MHz=(42.56) (STATIC FIELD STRENGTH IN TESLA). There are also phosphorous and other types of scanners wherein the Lamor equation would be different. The present invention applies to all such scanners.
The third type of electromagnetic field is the time-varying magnetic gradient fields designated BX, BY, BZ, which are used for spatial localization. These change their strength along different orientations and operating frequencies on the order of 1 kHz. The vectors of the magnetic field gradients in the X, Y and Z directions are produced by three sets of orthogonally positioned coils and are switched on only during the measurements. In some cases, the gradient field has been shown to elevate natural heart rhythms (heart beat). This is not completely understood, but it is a repeatable phenomenon. The gradient field is not considered by many researchers to create any other adverse effects.
It is instructive to note how voltages and electro-magnetic interference (EMI) are induced into an implanted lead system. At very low frequency (VLF), voltages are induced at the input to the cardiac pacemaker as currents circulate throughout the patient's body and create voltage drops. Because of the vector displacement between the pacemaker housing and, for example, the tip electrode, voltage drop across the resistance of body tissues may be sensed due to Ohms Law and the circulating current of the RF signal. At higher frequencies, the implanted lead systems actually act as antennas where voltages (EMFs) are induced along their length. These antennas are not very efficient due to the damping effects of body tissue; however, this can often be offset by extremely high power fields (such as MRI pulsed fields) and/or body resonances. At very high frequencies (such as cellular telephone frequencies), EMI signals are induced only into the first area of the lead system (for example, at the header block of a cardiac pacemaker). This has to do with the wavelength of the signals involved and where they couple efficiently into the system.
Magnetic field coupling into an implanted lead system is based on loop areas. For example, in a cardiac pacemaker unipolar lead, there is a loop formed by the lead as it comes from the cardiac pacemaker housing to its distal tip, for example, located in the right ventricle. The return path is through body fluid and tissue generally straight from the tip electrode in the right ventricle back up to the pacemaker case or housing. This forms an enclosed area which can be measured from patient X-rays in square centimeters. Per ANSI/AAMI National Standard PC69, the average loop area is 200 to 225 square centimeters. This is an average and is subject to great statistical variation. For example, in a large adult patient with an abdominal implant, the implanted loop area is much larger (around 400 square centimeters).
Relating now to the specific case of MRI, the magnetic gradient fields would be induced through enclosed loop areas. However, the pulsed RF fields, which are generated by the body coil, would be primarily induced into the leadwire system by antenna action. Subjected to RF frequencies, the lead itself can exhibit complex transmission line behavior.
At the frequencies of interest in MRI, RF energy can be absorbed and converted to heat. The power deposited by RF pulses during MRI is complex and is dependent upon the power (Specific Absorption Rate (SAR) Level) and duration of the RF pulse, the transmitted frequency, the number of RF pulses applied per unit time, and the type of configuration of the RF transmitter coil used. The amount of heating also depends upon the volume of tissue imaged, the electrical resistivity of tissue and the configuration of the anatomical region imaged. There are also a number of other variables that depend on the placement in the human body of the AIMD and its associated leadwire(s). For example, it will make a difference how much EMF is induced into a pacemaker lead system as to whether it is a left or right pectoral implant. In addition, the routing of the lead and the lead length are also very critical as to the amount of induced current and heating that would occur. Also, distal tip design is very important as the distal tip itself can heat up due to MRI RF induced eddy currents. The cause of heating in an MRI environment is twofold: (a) RF field coupling to the lead can occur which induces significant local heating; and (b) currents induced between the distal tip and tissue during MRI RF pulse transmission sequences can cause local Ohms Law heating in tissue next to the distal tip electrode of the implanted lead. The RF field of an MRI scanner can produce enough energy to induce RF voltages in an implanted lead and resulting currents sufficient to damage some of the adjacent myocardial tissue. Tissue ablation (destruction resulting in scars) has also been observed. The effects of this heating are not readily detectable by monitoring during the MRI. Indications that heating has occurred would include an increase in pacing threshold, venous ablation, Larynx or esophageal ablation, myocardial perforation and lead penetration, or even arrhythmias caused by scar tissue. Such long term heating effects of MRI have not been well studied yet for all types of AIMD leadwire geometries. There can also be localized heating problems associated with various types of electrodes in addition to tip electrodes. This includes ring electrodes or pad electrodes. Ring electrodes are commonly used with a wide variety of implanted devices including cardiac pacemakers, and neurostimulators, and the like. Pad electrodes are very common in neurostimulator applications. For example, spinal cord stimulators or deep brain stimulators can include a plurality of pad electrodes to make contact with nerve tissue. A good example of this also occurs in a cochlear implant. In a typical cochlear implant there would be sixteen pad electrodes placed up into the cochlea. Several of these pad electrodes make contact with auditory nerves.
Although there are a number of studies that have shown that MRI patients with active implantable medical devices, such as cardiac pacemakers, can be at risk for potential hazardous effects, there are a number of reports in the literature that MRI can be safe for imaging of pacemaker patients when a number of precautions are taken (only when an MRI is thought to be an absolute diagnostic necessity). While these anecdotal reports are of interest, they are certainly not scientifically convincing that all MRI can be safe. For example, just variations in the pacemaker leadwire length can significantly affect how much heat is generated. A paper entitled, HEATING AROUND INTRAVASCULAR GUIDEWIRES BY RESONATING RF WAVES by Konings, et al., Journal of Magnetic Resonance Imaging, Issue 12:79-85 (2000), does an excellent job of explaining how the RF fields from MRI scanners can couple into implanted leadwires. The paper includes both a theoretical approach and actual temperature measurements. In a worst-case, they measured temperature rises of up to 74 degrees C. after 30 seconds of scanning exposure. The contents of this paper are incorporated herein by reference.
The effect of an MRI system on the function of pacemakers, ICDs, neurostimulators and the like, depends on various factors, including the strength of the static magnetic field, the pulse sequence, the strength of RF field, the anatomic region being imaged, and many other factors. Further complicating this is the fact that each patient's condition and physiology is different and each manufacturer's pacemaker and ICD designs also are designed and behave differently. Most experts still conclude that MRI for the pacemaker patient should not be considered safe.
It is well known that many of the undesirable effects in an implanted lead system from MRI and other medical diagnostic procedures are related to undesirable induced EMFs in the lead system and/or RF currents in its distal tip (or ring) electrodes. This can lead to overheating of body tissue at or adjacent to the distal tip.
Distal tip electrodes can be unipolar, bipolar and the like. It is very important that excessive current not flow at the interface between the lead distal tip electrode and body tissue. In a typical cardiac pacemaker, for example, the distal tip electrode can be passive or of a screw-in helix type as will be more fully described. In any event, it is very important that excessive RF current not flow at this junction between the distal tip electrode and for example, myocardial or nerve tissue. This is because tissue damage in this area can raise the capture threshold or completely cause loss of capture. For pacemaker dependent patients, this would mean that the pacemaker would no longer be able to pace the heart. This would, of course, be life threatening for a pacemaker dependent patient. For neurostimulator patients, such as deep brain stimulator patients, the ability to have an MRI is equally important.
A very important and life-threatening problem is to be able to control overheating of implanted leads during an MRI procedure. A novel and very effective approach to this is to install parallel resonant inductor and capacitor bandstop filters at or near the distal electrode of implanted leads. For cardiac pacemaker, these are typically known as the tip and ring electrodes. One is referred to U.S. Pat. No. 7,363,090; US 2007/0112398 A1; US 2008/0071313 A1; US 2008/0049376 A1; US 2008/0024912 A1; US 2008/0132987 A1; and US 2008/0116997 A1, the contents of all of which are incorporated herein. Referring now to US 2007/0112398 A1, the invention therein relates generally to L-C bandstop filter assemblies, particularly of the type used in active implantable medical devices (AIMDs) such as cardiac pacemakers, cardioverter defibrillators, neurostimulators and the like, which raise the impedance of internal electronic or related wiring components of the medical device at selected frequencies in order to reduce or eliminate currents induced from undesirable electromagnetic interference (EMI) signals.
U.S. Pat. No. 7,363,090 and US 2007/0112398 A1 show resonant L-C bandstop filters to be placed at the distal tip and/or at various locations along the medical device leadwires or circuits. These L-C bandstop filters inhibit or prevent current from circulating at selected frequencies of the medical therapeutic device. For example, for an MRI system operating at 1.5 Tesla, the pulse RF frequency is 64 MHz, as described by the Lamour Equation for hydrogen. The L-C bandstop filter can be designed to resonate at or near 64 MHz and thus create a high impedance (ideally an open circuit) in the lead system at that selected frequency. For example, the L-C bandstop filter, when placed at the distal tip electrode of a pacemaker lead, will significantly reduce RF currents from flowing through the distal tip electrode and into body tissue. The L-C bandstop filter also reduces EMI from flowing in the leadwires of a pacemaker, for example, thereby providing added EMI protection to sensitive electronic circuits.
Electrically engineering a capacitor in parallel with an inductor is known as a bandstop filter or tank circuit. It is also well known that when a near-ideal L-C bandstop filter is at its resonant frequency, it will present a very high impedance. Since MRI equipment produces very large RF pulsed fields operating at discrete frequencies, this is an ideal situation for a specific resonant bandstop filter. Bandstop filters are more efficient for eliminating one single frequency than broadband filters. Because the L-C bandstop filter is targeted at this one frequency, it can be much smaller and volumetrically efficient.
A major challenge for designing an L-C bandstop filter for human implant is that it must be very small in size, biocompatible, and highly reliable. Coaxial geometry is preferred. The reason that coaxial is preferred is that implanted leads are placed at locations in the human body primarily by one of two main methods. These include guide wire lead insertion. For example, in a cardiac pacemaker application, a pectoral pocket is created. Then, the physician makes a small incision between the ribs and accesses the subclavian vein. The pacemaker leadwires are stylus guided/routed down through this venous system through the superior vena cava, through the right atrium, through the tricuspid valve and into, for example, the right ventricle. Another primary method of implanting leads (particularly for neurostimulators) in the human body is by tunneling. In tunneling, a surgeon uses special tools to tunnel under the skin and through the muscle, for example, up through the neck to access the Vagus nerve or the deep brain. In both techniques, it is very important that the leads and their associated electrodes at the distal tips be very small. US 2007/0112398 A1 solves these issues by using miniature coaxial or rectilinear capacitors that have been adapted with an inductance element to provide a parallel L-C bandstop filter circuit.
Prior art capacitors used in design of bandstop filters typically consist of ceramic discoidal feedthrough capacitors and also single layer and multilayer tubular capacitors and multilayer rectangular capacitors, and thick-film deposited capacitors. US 2007/0112398 A1 shows design methodologies to adapt all of these previous tubular, feedthrough or rectangular technologies to incorporate a parallel inductor in novel ways. It will be obvious to those skilled in the art that a number of other capacitor technologies can be adapted. This includes film capacitors, glass capacitors, tantalum capacitors, electrolytic capacitors, stacked film capacitors and the like.
As previously mentioned, the value of the capacitance and the associated parallel inductor can be adjusted to achieve a specific resonant frequency (SRF). The bandstop filters described in US 2007/0112398 A1 can be adapted to a number of locations within the overall implantable medical device system. That is, the L-C bandstop filter can be incorporated at or near any part of the medical device implanted lead system or at or adjacent to the distal tip electrodes. In addition, the L-C bandstop filter can be placed anywhere along the implanted lead system.
The L-C bandstop filters are also designed to work in concert with an EMI filter which is typically used at the point of leadwire ingress and egress of the active implantable medical device. For example, see U.S. Pat. No. 5,333,095; U.S. Pat. No. 5,905,627; U.S. Pat. No. 5,896,627; and U.S. Pat. No. 6,765,779, the contents of all being incorporated herein by reference. All four of these documents describe low pass EMI filter circuits. Accordingly, the L-C bandstop filters, as described in U.S. Pat. No. 7,393,090, are designed to be used in concert with such low pass filters.
When the value of a hermetic feedthrough filter capacitor is too high, the leading edge of MRI gradient pulse sequences can create an R-C charging circuit. As the feedthrough capacitor charges up this voltage fall can create one of two problems. First, the voltage induced on the leadwire system could directly capture the heart thereby creating a dangerously rapid heart rate which could then result in a dangerous ventricular arrhythmia. For example, ventricular fibrillation can result in sudden death. Another problem associated with too high of a value of a feedthrough capacitor at the input to the AIMD is that this R-C charging circuit can cause pulses to appear at the input sense amplifier (such as a cardiac pacemaker) such that the pacemaker would oversense or falsely interpret this input as a normal heartbeat. In certain cases this can cause a demand pacemaker to inhibit (stop pacing). For a pacemaker dependent patient this can lead to systole and be immediately life threatening. Accordingly, it is desirable for magnetic resonance compatibility to keep the value of the feedthrough capacitor relatively low (in the order of 1000 picofarads). On the other hand, in order to adequately protect AIMD device electronics from the powerful RF pulse field of MRI, we have a trade off in that it would be desirable to have the hermetic feedthrough capacitor be as large as value as possible (in the order of 4,000 to 6,000 picofarads).
When one performs MRI testing on an active implantable medical device (AIMD) with its associated lead system, one first establishes a controlled measurement. That is, with worst-case MRI equipment settings and a worst-case location within the MRI bore, and a worst-case lead configuration, one can measure heating using fiber optic probes at the distal electrodes. Temperature rises of 30 to over 60 degrees C. have been documented. When one takes the same control lead and places miniature bandstop filters in accordance with U.S. Pat. No. 7,363,090 or US 2007/0112398 A1, one finds that the distal electrodes are substantially cooled. In fact, in many measurements made by the inventors, temperature rises of over 30 degrees C. have been reduced to less than 3 degrees C. However, a secondary problem has been discovered. That is, the implanted lead acts very much as like a transmission line. When one creates a very high impedance at the distal electrode to tissue interface by installation of a resonant bandstop filter as described in U.S. Pat. No. 7,038,900 and as further described in US 2007/0112398 A1, there is created an almost open circuit which is the equivalent of an unterminated transmission line. This causes a reflection of MRI induced RF energy back towards the AIMD (for example, toward the pacemaker housing). This energy can be reflected back and forth resulting in temperature rises along the lead. In some cases, the inventors have measured temperature rises immediately proximal to the bandstop filters, which is undesirable.
Accordingly, there is a need for controlling the induced energy in implanted lead system. This may be accomplished by taking a system approach and carefully balance the filtering needs. Moreover, there is a need for novel tuned RF diverting circuits coupled to one or more energy or heat dissipation surfaces, which are frequency selective and are constructed of passive components. Such circuits are needed to prevent MRI induced energy from reaching the distal tip electrode or its interface with body tissue. By redirecting said energy to an energy dissipation surface distant from the distal electrodes, this minimizes or eliminates hazards associated with overheating of said lead and/or its distal electrodes during diagnostic procedures, such as MRI. Frequency selective diverter circuits are needed which decouple and transfer energy which is induced onto implanted leads from the MRI pulsed RF field to an energy dissipating surface. In this regard, a novel system is needed which can utilize the conductive housing (can) of the AIMD itself as the energy dissipation surface. A switched diverter circuit would be beneficial in such a system for minimizing heating of an implanted lead in a high power electromagnetic field environment. The present invention fulfills these needs and provides other related advantages.